This invention relates generally to measuring a motor speed for an induction motor, and specifically for measuring motor speed using a sensing coil to determine a rotor frequency.
The rotor frequency is used with a stator frequency to calculate the motor speed. In a two pole motor, for example, such as used for X-ray tubes, the speed (in rotations per minute) is equal to sixty times the difference between the stator and rotor frequencies (in hertz). The stator frequency is easily obtainable from the external leakage flux or the stator current spectrum. The determination of rotor frequency has been more difficult.
Flux sensing coils used to measure rotor frequency must often be placed far from the motor, sometimes with metal casings intervening. Additionally, if the motor is mounted on a rotating platform, the Earth's field may give rise to confusion in the signals.
Flux sensing coils have been used to measure motor speed via rotor leakage flux, as described, for example, in commonly-assigned U.S. Pat. Nos. 5,049,815 and 4,761,703; in other U.S. Pat. Nos. 534,596, 3,943,356, 4,839,585, and 5,530,343; in J. Penman et al., "Condition monitoring of electrical drives," IEEE Proceedings, Vol. 133, Part B, No. 3, May 1986; and
M. S. Erlicki et al., "Leakage Field Changes of an Induction Motor and Indication of Nonsymmetric Supply," IEEE Transactions on Industry and General Applications, vol. IGA-7, no. 6, November/December 1971. But these approaches typically require intrusive mounting of the sensing coil inside the motor housing, or are prone to errors and external signal interference due to their methods of measuring speed.